Focusing and cooling apparatus for electron beam tubes



y 1966 R. L. FENTON ETAL 3,259,787

FOGUSING AND COOLING APPARATUS FOR ELECTRON BEAM TUBES Filed March 26, 1962 3 a I 2s- 4f l l 2 l i l A 53 INVENTOR RICHARD 1.. FENTON SAUL SCHWARTZ United States Patent 3,259,787 FOCUSING AND COOLING APPARATUS FOR ELECTRON BEAM TUBES Richard L. Fenton, San Jose, and Saul Schwartz, Sunnyvale, Calif assignors, by mesne assignments, to Varian Associates, a corporation of California Filed Mar. 26, 1962, Ser. No. 182,195 9 Claims. ((11. 3153.5)

This invention relates to beam tubes as traveling wave tubes, and more particularly to apparatus for focusing the electron beam projected through such tubes.

In another of its aspects, the invention relates to a novel collector-cooler structure for a traveling wave tube.

Until recently, focusing of the electron beam in a traveling wave tube was effected by a large and relatively heavy solenoid adapted to provide a continuous magnetic field axially of the envelope. One of the advantages of this type of focusing structure is that the magnetic field which controls the electron beam is continuous from one end of the traveling wave tube to the other. One of the disadvantages is that the solenoid is heavy and requires a separate power source.

In more recent years it has become the practice to utilize permanent magnets to provide periodic permanent magnetic focusing of electron beams in beam tubes. These magnets are preferably of the annular wafer type which surround the body of the traveling wave tube and which are spaced apart by suitable pole pieces. The wafers are magnetized in an axial direction so that opposite faces are polarized in the opposite sense, and the wafers are arranged in the stack so that like poles abut a common pole piece. One of the advantages of this type focusing structure is the substantial reduction in weight of the magnet assembly and the elimination of extraneous power supplies. One of the disadvantages of the periodic permanent magnet focusing system is the difiiculty of applying the system to a traveling wave tube equipped With input and output waveguide structures without causing an undesirable discontinuity in the magnetic field at the occurrence of such input and output waveguide structures. Such discontinuities in the magnetic field cause the beam diameter at these points to vary. Accordingly, it is one of the objects of the invention to provide a periodic permanent magnetic focusing system which eliminates such discontinuities in the periodic magnetic field at the occurrence of the input and output waveguide structures.

Because of the extremely sensitive nature of a traveling Wave tube, and the difiiculty with which a traveling wave tube is normally focused, the custom has grown in the industry to tailor a magnetic circuit for each individual traveling wave tube, and it has heretofore been impractical if not impossible to utilize one magnetic circuit for a multiplicity of different tubes, the different tubes merely being plugged into the magnetic structure. It is therefore another important object of the present invention to provide a magnetic focusing structure for traveling wave tubes which permits theextraction of one traveling wave tube from the magnetic structure and the immediate insertion of another traveling wave tube, with provision for making minor adjustments of the position of the newly inserted tube so that the axis of the beam projected thereby exactly coincides with the axis of the magnetic field.

One of the characteristics of traveling wave tubes that has mitigated against a structure such as the one described has been the necessity to dissipate large amounts of heat from the collector electrode of the tube. Such heat dissipation has usually been effected in the past through fintype coolers attached permanently to the traveling wave tube collector. At least in one instance, the collectorcooler structure has been rendered removable by means of a tapered construction disclosed and claimed in United States Patent 2,958,797, assigned to the assignee of the present invention. However, in that structure it has been found that once the cooler is attached by the means therein shown, it is difficult to remove the cooler structure without damaging the tube because of the inherent jamming tendencies of the shallow tapers involved. It is accordingly another important object of the present invention to provide a cooler structure for a traveling wave tube collector which may be easily secured and removed from the collector.

Anotherimportant object of the invention is the provision of a magnetic structure for a traveling wave tube which permits readily releasable clamping of the cooler structure to the traveling wave tube collector, and the subsequent transverse adjustment of the traveling wave tube and collector-cooler structure to align the axis of the electron beam with the axis of the periodic permanent magnetic field.

Another object of the invention is the provision of a traveling wave tube magnetic focusing apparatus which may be fabricated by mass production techniques, and which may be assembled by relatively unskilled personnel, thus reducing the initial cost and replacement cost of magnetic structures.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood, however, that the invention is not limited to the embodiment illustrated and described, but may be incorporated in various forms within the scope of the appended claims.

Broadly considered, the invention in one of its aspects comprises a novel arrangement of periodic permanent magnets at the occurrence of the input and output radiofrequency waveguide structures normally associated with a traveling wave tube at points adjacent the input and out put ends of the slow wave structure therein. Such waveguide input and output structures are normally more than half a period thick at the point where they surround the traveling wave tube envelope and therefore constitute or cause a discontinuity to appear in the magnetic field which is formed by an axially aligned series of uniformly spaced periodic permanent magnets. It has been discovered that if the permanent magnet next adjacent each side of the waveguide is increased in diameter to about the width of the waveguide, and if a further magnet of parallelepiped form such as a bar is placed along each edge of the waveguide, that the discontinuity in the magnetic field at the occurrence of the waveguide substantially disappears. Enclosing the aligned series of periodic permanent magnets is a nonmagnetic housing fabricated in three portions which may be designated an input portion, a radiofrequency body portion and a collector portion, the portions being axially aligned and securely fastened to each other in a manner to maintain the integrity of the focusing structure enclosed thereW-ithin. Means are provided on the nonmagnetic housing to clamp a traveling wave tube in position therewithin, and means are also carried by the housing to properly align the axis of the electron beam generated by the traveling wave tube with the axis of the periodic permanent magnet focusing system. To secure efiicient heat dissipation from the collector of the traveling wave tube, a cooler structure is wedged onto the collector in a very tight manner. This ordinarily would make the cooler structure difficult to remove, thus endangering the integrity of the tube envelope which is notably quite fragile, and ejector means are therefore incorporated on the cooler structure for safely overcoming such jamming tendencies.

Referring to the drawings:

FIGURE 1 is a plan view of the traveling wave tube apparatus in assembled form. Portions of the structure are broken away to reveal the internal construction and to reduce the size of the figure.

FIGURE 2 is a side elevational view taken in the direction indicated by the arrow 2 in FIGURE 1.

FIGURE 3 is a vertical cross-sectional view of a portion of the structure taken in the plane indicated by the line 3-3 in FIGURE 1.

FIGURE 4 is a vertical cross-sectional view of another portion of the structure taken in the plane indicated by the line 4-4 in FIGURE 1.

FIGURES 1 and 2 are drawn approximately one-half actual size, and FIGURES 3 and 4 are drawn approximately actual size.

In more specific detail, the structure embodying the invention comprises a permanent periodic magnet stack comprising axially alinged annular magnetic wafers 2, separated by annular ferromagnetic pole piece plates 3, having on their inner peripheries cylindrical flange extensions 4. The cylindrical extension on each pole piece 4 underlies two adjacent magnetic wafers, and two adjacent extensions provide a gap 6 therebetween. The inner diameters of the cylindrical extensions 4 are pro portioned to fit around the R-F section 7 of a traveling wave tube provided with a gun section 8 of somewhat larger diameter than the radio-frequency section. The traveling wave tube envelope is preferably fabricated from glass, and in the figures, particularly FIGURES 3 and 4, the traveling wave tube is shown only in outline inasmuch as the construction of the traveling wave tube does not form part of the present invention.

In traveling wave tubes of this type in which the radiofrequency input or output signals are coupled onto or off the slow wave structure of the traveling wave tube by a waveguide portion 9 as shown in FIGURE 1, each waveguide extends perpendicular to the axis of the tube and normally displaces one or more of the annular wafer magnets 2. This displacement of the magnets causes a discontinuity in the periodic magnetic field at the occurrence of the waveguide, which is deleterious to the focusing of the beam. To eliminate such discontinuity, the present invention provides annular periodic wafer magnets 12 of somewhat larger diameter than the magnets 2, one of such magnets of enlarged diameter being positioned next adjacent opposite sides of the waveguide. Each of the magnets 12 is provided on the side thereof next adjacent the next smaller magnet with a plate-like ferromagnetic pole piece 13, having an outer diameter substantially equal to the diameter of the magnet 12, and an inner diameter matching the inner diameter of pole pieces 3. Pole pieces 13 are also provided with sym metrical extensions 14 on their inner peripheries as shown. On the side of each of the large diameter magnets 12 adjacent the waveguide, each magnet is provided with a substantially rectangular plate-like ferromagnetic pole piece 16, having a portion 17 extending radially beyond or past the waveguide.

The portions 17 serve to accommodate a bar magnet 18 therebetween. As shown in FIGURE 1, the bar magnet 18 extends perpendicular to the axis of the periodic magnet stack, and cooperates with the pole pieces 16 and the oversize magnets 12 to eliminate the discontinuity in the periodic magnet field at the occurrence of the waveguide. It should be noted that to accomplish this purpose each of the bar magnets is polarized perpendicular to its long axis so that its broadfaces are polarized in an opposite sense from each other. Each of the ferromagnetic pole pieces 16 is provided on its inner periphery with a cylindrical flange extension 19, the adjacent ends of the flange extensions defining a gap 21, providing a coupling slot coupling the waveguide to the slow wave structure within the traveling wave tube. The extensions 19 also function to lock the wave guide sections to the ferromagnetic pole pieces 16 so as to prevent transverse movement of the waveguide once the assembly has been completed.

As shown best in FIGURE 4, the initial ferromagnetic pole piece 22 next adjacent the electron gun 8 is substantially the same diameter as the ferromagnetic pole pieces 13, and is provided with a coaxially extending ferromagnetic shield cylinder 23 adapted to surround the electron gun. This ensures that the electrostatic focusing elements of the electron gun are shielded from the effects of the magnetic field generated by the periodic permanent magnet focusing stack. The shield cylinder 23 is preferably somewhat smaller in diameter than the pole piece 22 to provide a radially outwardly extending annular pole piece extension 24 projecting beyond the shield. Where desired, this annular extension may be provided with radially extending slots to provide the annular portion 24 with a measure of resilience.

At the output end of the traveling wave tube, the output waveguide 26, as shown best in FIGURE 3, is surrounded by substantially the same permanent magnet and magnetic pole piece structure, and like numbers have been assigned to like structure.

To clamp the periodic permanent magnet focusing stack comprising permanent magnets and pole pieces together into one composite unit, the housing structure is provided having a cylindrical gun section 31., having at one end a rectangular flange 32, the inner face 33 of which is adapted to lie snugly against the surface of pole piece extension 17. Integral bosses 34 extending from the face of flange 32 abut against the face 36 of a rectangular flange 37 formed integrally on the end of a tubular housing portion 38 proportioned to surround the periodic permanent magnet focusing stack between the waveguides 9 and 26. A similar flange 39 having a face 41 is formed on the opposite end of the tubular housing portion 38.

As seen in FIGURE 1, the faces 33 and 36 on flanges 32 and 37, respectively, abut closely against the pole piece extensions 17 and bind these pole pieces to the interposed magnet 18. Additionally, the bosses 34 extending toward flange 37 pass across the ends of the bar magnets 18 and the edges of pole pieces 16 so that when flanges 32 and 37 are bound together by screws 42, the bar magnets 18 lie trapped between the pole piece extensions 17 and the flange 32 and 37. To complete the assembly, a cover plate 43 is provided secured across opposite edges of the flanges by screws 44. As seen in FIGURE 4, it will be understood that two such radially spaced magnets 18 are provided as shown.

The opposite end of the periodic permanent magnet stack is secured in the same manner between the flange 39 and a collector housing portion 46 having a face portion 47 closely abutting the adjacent magnetic pole piece 17, and having axially extending integral bosses 48 abutting the face 41 of flange 39. The bar magnets 18 at this end of the structure are thus likewise trapped by the pole piece extensions 17 between flange faces 41 and 47, the axially extending integral bosses 48, and a cover plate 49 secured by screws 51. The collector housing assembly 46 is secured to flange 39 by cap screws 52. It will thus be seen that with this construction the periodic permanent magnet stack lies trapped between the three housing portions 31, 38 and 46, and that the waveguide sections 9 and 26 likewise lie locked to the magnetic structure. Temperature compensating strips 53 circumferentially spaced about the magnet-pole piece stack between pole pieces 13 adjacent opposite ends of the stack and bound thereon by any appropriate means lend additional rigidity to the assembly.

Because minor discrepancies are inevitably made in the assembly of different traveling wave tubes of the same design, it is essential that some means he provided for aligning the axis of the electron beam projected through each traveling wave tube inserted into the magnetic structure with the axis of the magnetic structure. For this purpose, the electron gun housing portion of the magnetic structure is provided with an enlarged internal diameter portion 53, within which is arranged an alignment ring 54 of somewhat smaller outside diameter to permit transverse shiftability of the alignment ring. At one end the alignment ring is adapted to abut snugly against a shoulder 56 formed on the internal surface of the housing member 31, the internal diameter of which is such that the housing member 31 fits snugly around the shield member 23 and thus aids in aligning the stack within the housing. A radially inwardly extending flang 57 formed on the alignment ring intermediate its ends provides a seat or shoulder 58 within which is seated a collar 59 fixed about the electron gun end of the traveling wave tube. The collar is fixed on the traveling wave tube at the factory and on each tube provides a reference plane from which the focal point of the electron beam is spaced a known amount. A gland 61, preferably of synthetic resin, threaded onto the end of the housing portion 31 abuts against the collar 59 and urges the traveling wave tube into position within the magnetic circuit. When the collar is seated against the alignment ring, the focal point of the beam will be exactly positioned with respect to the periodic permanent magnet stack. The gland 61 retains the traveling wave tube so positioned against axial movement with relation to the magnetic circuit.

To effect transverse adjustment of this end of the traveling wave tube, the magnetic circuit is provided with a plurality of adjustable thumb screws 62, preferably of synthetic resin, the inner ends of which abut against the bottom 63 of a groove 64 formed in the outer periphery of the alignment ring. Two thumb nuts 90 apart are preferably provided, and opposite each of the thumb nuts is a spring loaded pin 66 urged radially inwardly against the alignment ring by a coil spring 67 interposed between the hollow interior of pin 66 and a cap 68 threadedly secured to gun housing portion 31. It will thus be seen that by a very small movement of either or both thumb screws 62, the gun end of the traveling wave tube may be shifted in any transverse direction against the spring pressure exerted by coil spring 67. At the collector end of the structure, a somewhat similar alignment ring or collet 71 is provided having spring fingers 72 thereon forming a snug sliding fit on the body of the rtaveling wave tube adjacent the collector. Thumb screws 73, similar to the screws 62 at the other end of the device, are adjustable to move the collet transversely with respect to the magnet stack, such movement resulting in the collet carrying the collector end of the traveling wave tube with it. The thumb screws at this end of the device work against spring loaded pins 74, each of which is urged inwardly by a coil spring 76 retained in place by a set screw 77 threaded into the collector housing portion 46. It will thus be seen that with this arrangement, the traveling wave tube which is plugged into the magnetic circuit can be closely adjusted to coincide the axis of the electron beam with the axis of the periodic permanent magnet focusing stack. An indication that the electron beam is perfectly aligned with the axis of the magnetic structure may be obtained by an appropriate gauge connected to measure the collector current.

Since the collector of a traveling wave tube is integral with the tube body, and since the entire structure must be inserted through the relatively small diameter of the periodic permanent magnet stack, it is essential that the collector 81 be provided with a detachable cooler structure designated generally by the numeral 82, which may be easily attached and detached from the collector, while still providing the requisite thermal conductance between the collector surface and the radiating surfaces of the cooler structure. To achieve these results there is provided a collet 83 having a cylindrical interior surface 84 to match the exterior surface of the collector 81, but having a tapered exterior surface 86 converging toward one end of the collector.

The collet is preferably divided into a plurality of circumferentially spaced segments 87, separated from each other by longitudinally extending slots 88 as shown best in FIGURE 3. The collet is adapted to slip snugly around the collector, and is provided with a threaded extension 89 adapted to receive a thumb nut 91 having a radially extending flange 92 thereon in abutting relation with one end of a central hub 93 'having an interior surface 94 provided with a taper matching the taper on the collet 81 and a plurality of radially extending fins 96 on its outer surface.

Thus, when the collet 81 is moved longitudinally with respect to the hub 93, collet segments 87 are urged into intimate and thermally conductive contact on the one side with the surface of the collector, and into intimate and thermally conductive contact on the other side with the interior of the hub 93. Movement of the collet relative to the hub may easily be effected by rotation of the thumb screw 91 so as to draw the two parts together. It has been found, however, that because the taper on the interior of the hub and on the exterior of [the collet is so shallow, there is a tendency for the cooler structure to bind on the collet, rendering it extremely diflicult to remove the cooler from a tube and therefore the tube from the magnetic assembly. This tendency has been overcome by trapping the flange 92 of thumb screw 91 so that rotation of the thumb screw in a counterclockwise direction will drive the collet from the hub.

The thumb screw flange is suitably trapped by the overlying bracket structure 97 secured to the cooler structure by screws '98. It has been further found that over-all relative movement between the hub and collet need be very slight, in the nature of about one-sixteenth of an inch, and that it is desirable to limit such movement. For this purpose, the threaded portion 89 of the collet is provided with a stop nut 99 which limits inward movement of the collet with respect to the hub. It will thus be seen that an easily attached and detached cooler structure has been provided which also provides intimate thermal contact between the collector surface and a relatively larger radiating cooler surface.

As shown in FIGURE 3, the collector of the traveling wave tube extends somewhat beyond the end of the magnetic housing portion 46 and, unless protected in some manner, is subject to being inadvertently struck with consequent misalignment of the collector portion of the tube with the radio-frequency portion of the tube. To prevent such inadvertent destruction of the tube, the collector and cooler structure is enclosed within a tubular housing shell 101 having enlarged apertures 102 therein for the passage of cooling air into and around the finned cooler structure. At one end the housing shell is internally threaded and secured to a cylindrical flange portion 104 integral with the outer periphery of a dielectric plate 106, the inner periphery of which overlaps the collet 71 to retain the collet trapped between plate 106 and the block 46. The plate 106 is suitably secured to the block by screws 107. -Tests have shown that the placement of such a shell about the collector-cooler assembly does not appreciably reduce the efficiency of the collector-cooler structure, but does prevent inadvertent destructive blows being delivered to the collector. In assembling the magnetic structure, the multiplicity of annular magnetic rings and pole pieces are suitably arranged on a mandrel and the stack so formed clamped tightly between the three housing portions 31, 38 and 46. With the mandrel in place it has been found that alignment of the stack and housing 'is complete when the screws 42 and 52 which draw the units together are tightened down. After being so clamped, the mandrel may be withdrawn and the periodic permanent magnet stack will remain clamped between the three housing portions. For this purpose the parts are proportioned so that the inner periphery of the housing portion 31 snugly surrounds the shield 23 formed on the initial pole piece 22 and abuts against the flange 24 (FIGURE 4), so that tightening of screws 42 and 52 urges the entire stack tightly against the collector housing portion 46, the innermost periphery of which fits snugly about the cylindrical extension of the last pole piece 3 as shown in FIGURE 3.

Cover plates 43 and 49 are next applied, and thereafter 7 the alignment rings 54 and 71 are applied, together with the adjustment screws 62 and 73 and their opposed spring loaded pins 66 and 74. The tube may now be inserted into the magnetic structure until the collar 59 on the electron gun seats in the seat 58 of alignment ring 54, after which the gland 61 may be applied to trap the tube Within the magnetic housing against axial displacement. Next, the cooler assembly is applied over the collector as previously discussed, and the thumb nut 91 turned up so as to clamp the cooler on the collector. The safety shell 101 is then suitably secured to plate 106. The tube may now be connected into the required power supplies, and suitable adjustments made .to the thumb screws 62 and 73 to align the electron beam of the tube exactly with the axis of the periodic permanent magnet stack. Such.

adjustment is apt to be extensive for the first tube inserted into the apparatus, but thereafter succeeding tubes should align themselves very closely with the settings previously established. Because of inherent manufacturing discrepancies, some small adjustment of. the screws 62 and 73 may be required, but generally, the second tube inserted into the structure may immediately be turned full on without destroying the tube. It will thus be apparent that a tube may be removed and replaced by another within a matter of minutes.

We claim:

1. In combination with a traveling wave tube having an electron gun section for generating and projecting an electron beam along an axis, a collector electrode for intercepting the beam, and a radio-frequency interaction section interposed therebetween and including a slow wave structure having radio-frequency input and output ends and Waveguides coupled thereto; an apparatus for focusing the traveling Wave tube inserted therein comprising, a periodic permanent magnet stack constituted of a series of juxtaposed alternately arranged annular permanent magnets and ferromagnetic pole pieces, adjacent magnets of said series having like poles facing each other and with said pole pieces forming a plurality of periodic magnetic fields having a common axis, the two pole pieces next adjacent opposite sides of each waveguide projecting diametrically beyond the outer peripheries of the next adjacent permanent magnets, auxiliary parallelepiped permanent magnets interposed between the diametric extensions of said two pole pieces, a nonmagnetic housing supporting said periodic permanent magnet stack and said auxiliary parallelepiped permanent magnets in fixed relation to said waveguides, and means on the housing to selectively position the traveling wave tube within the housing to align the axis of the electron beam with the axis common to said periodic magnetic fields.

2. The combination according to claim 1, in which the two permanent magnets next adjacent opposite sides of each waveguide have a diameter larger than the remaining permanent magnets.

3. The combination according to claim 1, in which the diametrically projecting portions of the pole pieces next adjacent each waveguide are substantially rectangular.

4. The combination according to claim 1, in which said auxiliary parallelepiped magnets are polarized perpendicular to their long sides and are arranged with their poles facing like poles of the next adjacent permanent magnets.

5. The combination according to claim 1, in which said means on the housing to selectively position the traveling wave tube includes an alignment ring mounted adjacent each end of the housing to support the associated end of the tube and adjustable to selectively transversely shift the traveling wave tube within the housing to align the axis of the electron beam with the axis common to said periodic magnetic fields.

6. The combination according to claim 1, in which said means on the housing to selectively position the traveling wave tube includes means to selectively retain the tube against axial displacement in relation to said periodic permanent magnet stack and the housing.

. 7. The combination according to claim 1, in which a cooler structure is detachably mounted on the collector Within the housing and is movable transversely with the tube in relation to the housing.

8. A focusing apparatus for a traveling wave tube having an electron gun section for generating and projecting an electron beam along an axis and a collector electrode for intercepting the beam together with a radio-frequency interaction section interposed therebetween and including a slow wave structure having radio-frequency input and output ends and Waveguides coupled thereto comprising, a nonmagnetic housing including an electron gun section, a radio-frequency interaction section and a collector section, said sections being detachably connected in axial alignment, a periodic permanent magnet stack fixed within the housing in substantially axial alignment therewith, said periodic permanent stack including a pair of laterally displaced auxiliary permanent bar magnets interposed between opposed lateral edges of each waveguide and the associated housing sections, and means on the housing to selectively position the traveling wave tube within the housing to align the axis of the electron beam with the axis common to the periodic magnetic fields of the periodic permanent magnet stack.

9. The combination according to claim 8, in which a cooler structure is detachably mounted on the collector for movement transversely with the tube in relation to the housing.

References Cited by the Examiner UNITED STATES PATENTS 2,413,179 12/1946 Grandmont et al. 185 2,419,234 4/1947 Holihan 165-l85 2,985,789 5/1961 St. John 3153.5 3,054,015 9/1962 Fujii 3153.5

DAVID J. GALVIN, Primary Examiner.

GEORGE N. WESTBY, Examiner.

S. SCHLOSSER, V. LAFRANCHI,

Assistant Examiners. 

1. IN COMBINATION WITH A TRAVELING WAVE TUBE HAVING AN ELECTRON GUN SECTION FOR GENERATING AND PROJECTING AN ELECTRON BEAM ALONG AN AXIS, A COLLECTOR ELECTRODE FOR INTERCEPTING THE BEAM, AND A RADIO-FREQUENCY INTERACTION SECTION INTERPOSED THEREBETWEEN AND INCLUDING A SLOW WAVE STRUCTURE HAVING RADIO-FREQUENCY INPUT AND OUTPUT ENDS AND WAVEGUIDES COUPLED THERETO; AN APPARATUS FOR FOCUSING THE TRAVELING WAVE TUBE INSERTED THEREIN COMPRISING, A PERIODIC PERMANENT MAGNET STACK CONSTITUTED OF A SERIES OF JUXTAPOSED ALTERNATELY ARRANGED ANNULAR PERMANENT MAGNETS AND FERROMAGNETIC POLE PIECES, ADJACENT MAGNETS OF SAID SERIES HAVING LIKE POLES FACING EACH OTHER AND WITH SAID POLE PIECES FORMING A PLURALITY OF PERIODIC MAGNETIC FIELD HAVING A COMMON AXIS, THE TWO POLE PIECES NEXT ADJACENT OPPOSITE SIDES OF EACH WAVEGUIDE PROJECTING DIAMETRICALLY BEYOND THE OUTER PERIPHERIES OF THE NEXT ADJACENT PERMANENT MAGNETS, AUXILIARY PARALLELEPIPED PERMANENT MAGNETS INTERPOSED BETWEEN THE DIAMETRIC EXTENSIONS OF SAID TWO POLE PIECES, A NONMAGNETIC HOUSING SUPPORTING SAID PERIODIC PERMANENT MAGNET STACK AND SAID AUXILIARY PARALLELEPIPED PERMANENT MAGNETS IN FIXED RELATION TO SAID WAVEGUIDES, AND MEANS ON THE HOUSING TO SELECTIVELY POSITION THE TRAVELING WAVE TUBE WITHIN THE HOUSING TO ALIGN THE ASXIS OF THE ELECTRON BEAM WITH THE AXIS COMMON TO SAID PERIODIC MAGNETIC FIELDS. 